How do we know what the wind strength is just by looking at the windsock?

As per Transport Canada standards, a windsock will extend horizontally in a wind of 15 knots or more. At 10 knots the windsock will be 5° below the horizontal. At 6 knots the windsock will be 30° below the horizontal. At 3 knots the windsock will orientate itself in the direction of the wind.

15 KT – horizontal
10 KT – 5° below horizontal
6 KT – 30° below horizontal
3 KT – orientates with wind direction